1. Field of the Invention
The present invention relates to a linear motor used in industrial machinery, such as a machine tool.
2. Description of the Related Art
Conventionally, linear motors have been used in industrial machinery, such as machine tools, as means for realizing speed enhancement and a high level of precision. One such linear motors is disclosed in Japanese Patent Laid-Open Publication No. 2002-238241, in which costly permanent magnets are placed on a slider; in particular, in a long-stroke machine, thereby minimizing use of the permanent magnets to thereby reduce motor cost.
FIG. 2A shows the conventional linear motor disclosed in the above Japanese Patent Laid-Open Publication No. 2002-238241.
FIG. 2B shows magnetized directions of permanent magnets in the conventional linear motor, and FIG. 3 shows connection of AC windings in the conventional linear motor. Specifically, in FIG. 2A, a stator 12 fixed, for example, on a bed of a machine tool is formed by laminating flat rolled magnetic steel sheets, and has stator salient poles 10 formed on a surface thereof at a pitch P. Further, a slider 11 fixed, for example, on a table of the machine tool is movably supported by a rolling guide or the like provided between the bed and the table of the machine tool along an X-axis direction depicted in FIG. 2A. Similar to the stator 12, the slider 11 is formed by laminating, for example, flat rolled magnetic steel sheets, and has U, V, and W phase teeth 13, 15, and 14, shifted from each other along the X-axis direction by P/3 corresponding to an electrical angle of 120 degrees. U, V, and W phase AC windings 16, 18, and 17 are wound around the teeth. A plurality of permanent magnets 19 are disposed in alternating sequence of N, S, N, S, . . . on the surface of the slider 11. Taking S and N as one pair, as shown in FIG. 2B, each of the teeth 13, 14, and 15 has three pairs of the permanent magnets 19 arranged thereon at the pitch P. Reference numeral 22 represents magnetic flux in a magnetic yoke 20 of the slider and a magnetic yoke 21 of the stator when a current is supplied to the AC windings 16, 17, and 18 in a direction from U to V and W. The AC windings 16, 18, and 17 are connected so as to establish a star connection of U, V, and W phases.
Here, when a current is applied to the AC windings 16, 17, and 18, the teeth 13, 14, and 15 constituting the three phases are excited in a positive or negative direction along a Y-axis direction depicted in FIG. 2A. At this time, magnetic flux of one portion of the permanent magnets 19 having a magnetic direction identical to the excitation direction of the AC windings 16, 17, and 18 is enhanced, while magnetic flux of the other portion of the permanent magnets 19 having a magnetic direction opposite the excitation direction is suppressed. As a result, the teeth 13, 14, and 15 are excited to exhibit either N or S polarity, thereby forming a large North or South magnetic pole. The magnetic flux 22 passing through the teeth 13, 14, and 15 and the stator forms a closed loop as shown in FIG. 2A. Therefore, the force of magnetic attraction in accordance with a position of the slider 11 in relation to the stator 12 is generated, thereby producing a thrust force of the slider 11.
A flow of the magnetic flux 22 will be described in detail below. When a current is applied in a direction from the U phase towards the V and W phases; in other words, when a current is fed through the AC winding 16 in an illustrated winding direction and fed through the AC windings 17 and 18 in a direction opposite the illustrated winding direction, the tooth 13 becomes a South pole, whereas the teeth 14 and 15 become North poles in FIG. 2A. Further, the magnetic flux 22 forms a magnetic path flowing from the tooth 13 to the teeth 14 and 15 and then returning through the stator 12 to the tooth 13. As a result, the force of magnetic attraction acts on the slider 11 in the X-axis direction, which produces the thrust force of the slider 11.
The conventional linear motor shown in FIGS. 2A and 2B is characterized by realizing cost reduction of the linear motor by disposing costly permanent magnets on the slider to thereby decrease use of the permanent magnets 19; in particular, in a case where the stroke is long. In addition, the linear motor has a characteristic such that a plurality of magnetic poles composed of the teeth 13, 14, and 15 and the plurality of permanent magnets 19 are excited by means of a single winding, thereby allowing the winding to become shorter, and, in turn, yielding an effect of reducing a loss due to electrical resistance occurring when currents pass through the winding; i.e., so-called copper loss, thereby realizing improved efficiency.
It should be noted that, as a method of connecting the AC windings, a delta connection may be adopted instead of the star connection illustrated in FIG. 3.
In a machine tool in which tables are moved via a feed rod actuated by a linear motor, an essential factor is that the linear motor should drive the tables with a constant thrust force to produce a smooth machined surface. To meet this need, thrust ripples of the linear motor must be minimized. However, the conventional linear motor shown in FIGS. 2A and 2B suffers problematic occurrence of a relatively large thrust ripple resulting from a positional relationship between front ends of the teeth 13, 14, and 15 in a moving direction thereof provided to the slider 11 and the stator salient poles 10 attached to the stator 12.
Because the teeth 13, 14, and 15 are disposed to be shifted by P/3 corresponding to 120 electrical degrees, movement of the slider 11 relative to the stator 12 causes one of the front ends of the teeth 13, 14, and 15 to pass through an edge of one of the stator salient poles 10 on the same side at an interval of P/3, bringing about a change in permeance. From this change in permeance, periodicity of occurrence of the thrust ripple is determined as P/3.
FIG. 4 shows another conventional linear motor disclosed in Japanese Patent Laid-Open Publication No. 2002-101636 adopting a structure for reducing the ripple having the P/3 periodicity.
In FIG. 4, an armature A has a structure similar to that of the slider 11 in the conventional linear motor shown in FIG. 2A, and an armature B is identical with the armature A except for the permanent magnets disposed on the U, V, and W phase teeth arranged in the order of N, S, N, . . . ; i.e., arranged so as to be opposite in direction of magnetic pole to the armature A. The armatures A and B are fixed on a clamping plate 23 in such a manner that positions of the armatures A and B relative to the stator salient poles are shifted by P/2; i.e., shifted by 180 degrees in electrical angle.
FIG. 5 shows a thrust ripple of the conventional linear motor of FIG. 4 configured as described above. Because the armatures A and B are displaced by P/2 corresponding to 180 electrical degrees, the trust ripple with P/3 periodicity occurring in the armature A and that occurring in the armature B have a relationship such that the thrust ripples are out of phase with each other by 180 degrees, but of the same amplitude. Accordingly, the thrust ripples with P/3 periodicity cancel each other. As can be seen from the relationship between the amount of displacement of the armatures A and B and the thrust ripple shown in FIG. 6, the thrust ripple can be reduced to one-fifth or less its original level, so long as the displacement is 160˜200 electrical degrees. For this reason, the armatures A and B are not necessarily displaced 180 electrical degrees with a high degree of precision, and a sufficient effect of reducing thrust ripple can be obtained by the displacement within the range of 160 to 200 electrical degrees.
Such a conventional linear motor as described above, however, has a problem that remains to be solved, which will be described below.
When a linear motor is used to drive a feed rod for a table in a machine tool, for example, the table should be driven smoothly with a uniform thrust force to produce a smooth machined surface; therefore, the thrust ripple occurring with P/3 periodicity must be minimized. Because the linear motor is embedded in a machine and used therein in general, the size of the linear motor is desirably reduced to the extent possible. However, because the conventional linear motor disclosed in Japanese Patent Laid-Open Patent No. 2002-101636 is constructed using the two armatures A and B, the length of the slider becomes longer in its moving direction, which poses difficulty in compact incorporation of the linear motor into the machine and causes extension, by the length of one slider, of the length of the stator, which is designed on the basis of the the stroke of a moving body+the length of the slider. Further, third and fourth embodiments described in the above patent document teach a layout in which the armatures A and B are displaced by P/2 corresponding to an electrical angle of 180 degrees and arranged on the slider in parallel with the slider moving direction. In this layout, the size of the slider is increased in a direction perpendicular to the slider moving direction, which also makes it difficult to install the linear motor in confined spaces of the machine.
In addition, another demand for a linear motor is weight reduction. In a machine tool, for example, a table driven by a liner motor is operated under increased acceleration and deceleration for the purpose of reducing machining time, and this raises a demand for further weight reduction. In the conventional linear motor, a force of magnetic attraction, which is several to ten times greater than the thrust force, is generated between the slider and the stator, along a direction perpendicular to the slider moving direction. The force of magnetic attraction along the direction perpendicular to the slider moving direction problematically deforms a structure for movably supporting the linear motor and/or a rolling guide, which results in lowered accuracy of machining a workpiece.